1. Field of the Invention
The present invention relates to an optical record medium which can record or erase information of a large capacity at a high density with use of a laser beam or the like.
2. Description of the Prior Art
A write-once type disk is known as an optical record memory, and it comprises a material which can transform from the crystalline phase to the amorphous phase such as TeO.sub.x (0&lt;.times.&lt;2.0) having Te and TeO.sub.2 as main components. In such an optical record memory, a data is written thermally by irradiating the optical disk with a laser beam or the like.
Further, a write erasable disk wherein information can be recorded and erased repeatedly has also been used practically as an optical record memory. Such a write erasable disk can record information by changing a mark point in a record thin film to the amorphous state with a laser beam to heat, melt and cool rapidly the record thin film, while it can erase information by changing a mark point in the record thin film to the crystalline state with a laser beam to heat and cool gradually the record thin film. Materials which can transform reversibly between the crystalline phase and the amorphous state are used for such a record thin film, and they include a chalcogenide material Ge.sub.15 Te.sub.81 S.sub.2 by S. R. Ovshinsky and the like, and As.sub.2 S.sub.3, As.sub.2 Se.sub.3, Sb.sub.2 Se.sub.3 and the like which are combinations of a chalcogen element with another element in group V such as As or in group IV such as Ge in the periodic table. An optical disk can be produced by forming such a record thin film on a substrate having grooves for guiding a laser beam. In order to record or erase information in such an optical disk with a laser beam, the record thin film is crystallized at first. Then, the intensity of a laser beam focussed at diameter of about 0.5 .mu.m on the rotated disk is modulated for irradiation in correspondence to information, so that the temperature of a point irradiated with the laser beam at a peak power is increased above the melting point of the record thin film and is cooled rapidly. Thus, the information is recorded as an amorphous mark. Further, the temperature of a point irradiated at a bias peak power rises above the crystallization temperature of the record thin film, so that the information recorded previously can be erased or an overwrite is possible.
As explained above, the temperature of the record thin film can be risen with a laser beam above the melting point or above the crystallization point. Therefore, heat-resistant dielectric layers are generally provided above and below the record thin film as protection layers for the substrate and for an adhesive layer. Because the characteristics of temperature rise, rapid cooling and gradual cooling vary with the thermal conduction characteristic of the protection layers, the recording and erasing characteristics depend on the materials selected for the protection layers.
As shown in FIG. 1, grooves 31 are provided in an optical disk and record marks 30 are formed along the grooves with a laser beam thermally. In the above-mentioned optical disks of the write erasable type and of the write once type, information can be recorded by rising the temperature of the record thin film above the melting point to transform the record thin film to the amorphous phase and by cooling the record thin film rapidly. On irradiation of a laser beam along a groove 31 to form a record mark 30, a temperature difference exists between the center line 32 and an edge 33 in a groove 31 in the groove 30. Then, if the power of the laser beam for erasing is small, the temperature of the edges 33 may not rise above the melting point and the edges 33 may be remained not to be transformed in the amorphous state. Then, a large recording power is needed for recording a mark, and the sensitivity of optical disk becomes worse.
Further, in a write erasable optical disk, information can be erased by rising the temperature above the crystallization temperature. If the power of the laser beam for erasing is small, the temperature of the edges 33 may not rise above the crystallization point and the edges 33 may be remained not to be erased. If the erasing power is increased for erasing the record mark 30 completely, the temperature difference increases and the temperature of the center line 32 increases. Thus, it is a problem that the margin of erasing power becomes narrow.